Sodium influx at the outer surface of frog skin : Evaluation of different extracellular markers

The unidirectional sodium influx across the outside surface of the frog skin epithelium was measured. The method was identical to the one described by Biber and Curran [4] except that mannitol instead of inulin was used as the indicator for the amount of tracer containing test solution remaining on the surface of the skin after blotting. The space of distribution for(3)H-mannitol is about twice as large as the corresponding space for inulin after 32-sec exposure to the tracer. At a sodium concentration of 6.7MM the sodium influx determined with inulin as marker for the test solution is 0.146 μEquiv hr(-1)cm(-2) larger than the influx measured in the same preparation with mannitol. This difference increases in proportion to the sodium concentration and can be ascribed to diffusion of sodium into a space which is accessible to mannitol but not to inulin during a 30-sec interval. Hence, nearly two-thirds of the previously described linear component of the sodium influx proceeds, as was suspected previously, into a compartment which is not directly connected to net sodium transport across the skin. The nature of the remaining one-third of this linear compartment is not clear but previous experiments suggest that it is also not involved in net sodium transport.